The present invention generally relates to composite membrane materials, and more particularly composite membrane materials formed by laminating multiple porous layers while maintaining a desired level of porosity within the composite membrane material.
Porous membranes are employed in a wide variety of applications, nonlimiting examples of which are microventing, liquid filtration, and microfiltration systems, including water filtration systems and filter bag media for dust collectors. Desirable properties of porous membranes will depend on the particular application, though generally controlled porosity, resistance to temperature, chemicals and/or abrasion, and wettability are often of particular interest. Porous membranes have been produced from various materials, including polypropylene, acrylics, polyesters, polyphenylene sulfide (PPS) such as Torcon® and Procon®, aramids such as Nomex®, polyimides such as P84, fiberglass, and polytetrafluoroethylene (PTFE) such as Teflon®. Of these, PTFE and particularly expanded PTFE (ePTFE) membranes have found wide use in view of its chemical resistance and porosity characteristics. The production of ePTFE generally entails extruding a tape formed of PTFE, and then subjecting the tape to biaxial stretching in the plane of the tape to produce a membrane containing pores, often micropores, i.e., pore sizes of less than one micrometer. Because PTFE is hydrophobic, treatment is required to allow the use of PTFE membranes for filtration applications in which water or a water-containing liquid is to be filtered. Various treatment techniques are well know for imparting hydrophobic, oleophobic and hydrophilic properties to PTFE membranes.
For certain applications, composite ePTFE membranes are constructed of two or more microporous ePTFE membranes that are laminated together with the assistance of a bonding agent. Suitable bonding agents are typically formed of polymeric materials having lower melting temperatures than PTFE to allow the bonding materials to melt and bond the PTFE membranes together without melting or otherwise damaging the membranes. The bonding agent may be applied in liquid form or as an adhesive web or film that is melted during the lamination process. However, the presence of the bonding agent between PTFE membranes inevitably reduces porosity of the composite membrane, particularly if the membranes are microporous.
In view of the above, it would be desirable if alternative methods were available by which membranes, and particularly microporous membranes formed of ePTFE, could be bonded together to form composite membrane materials.